Manufacturers may reduce carbs in food by changing the process or adding high protein foods – sometimes they substitute refined wheat flour with soy flour that has high soy protein content , or  wheat protein. They may add what bran or oat bran that has extra fiber content. they may add nuts which have healthy fats. They may also replace sugar with other substitutes such as sorbitol.

sometimes manufacturers may subtract the carbs from fiber and list effective carbs only. They claim that since fiber does not affect blood sugar they are allowed to discount the carbs from fiber.  you should clearly look at net impact carb calculations in detail to make sure you are not paying for similar item higher price because you think it is low carb.